1. Field
Embodiments of the present disclosure pertain to electrically conductive polymer compositions and, in particular, to surfacing and adhesive films formed from thermosetting polymer compositions that incorporate conductive additives.
2. Description of the Related Art
Polymer matrix composite structures (PMCs) are being increasingly used for aerospace applications. For example, PMCs have been employed in amounts up to about 50% in commercial aircraft. PMCs combine selectively oriented fibers that are enveloped in a surrounding polymeric matrix material. These composite structures exhibit good mechanical properties for their weight (e.g., strength, stiffness, toughness), as well as a wide service temperature window and ease of manufacture, making them well suited for aerospace applications.
Surfacing films, such as epoxy-based films, are often incorporated into polymer composites to provide the composites with the surface quality required for aerospace applications. For example, surfacing films may be co-cured with prepregs to provide a substantially porosity free surface that protects the underlying composite, while reducing labor, time, and cost of composite manufacturing.
Epoxy-based surfacing films, however exhibit poor resistance to electromagnetic energy (EME) events, such as lightning strike (LS), electrostatic discharge (ESD), and electromagnetic interference (EMI) due to their insulative properties. The relatively high resistivity exhibited by epoxies inhibits the energy of a lightning strike from dissipating adequately, resulting in skin puncture and delamination of the underlying composite structure. Further, the charge generated on the surface of the composite can remain for long time periods, elevating the risk of ESD in low relative humidity environments that can damage electronic systems and risk sparking in the vapor space of fuel tanks. Additionally, the poor electrical conductivity of epoxy-based surfacing films may inhibit the mobility of charge carriers, which can impair the ability of the composite structure to provide EMI shielding.
To minimize the effect of lightning strike on a composite structure, different ways of enhancing the conductivity of the composite structure have been used to provide LS/ESD/EMI protection for composite parts on aircraft. Examples of such approaches include solid or segmented diverters, arc-sprayed or flame-sprayed metals, woven wire meshes, solid/expanded/perforated foils, metal coated fiber cloths, interwoven wire fabric (IWWF) highly conductive fibers, and metal loaded conductive paints. In further examples, expanded metal screens (e.g., copper, aluminum mesh) have been embedded in surfacing films attached on a composite surface to dissipate the energy incurred by lighting strike for protection against such events.
Detrimentally, however, surfacing film systems with embedded metal screens (e.g., copper or aluminum, with fiberglass isolation layer) significantly increase the overall weight of the aircraft. Furthermore, integrating these surfacing film systems into composite materials may significant increase the materials and labor costs for the manufacture of the composite parts. Additionally, it may be difficult to interconnect these surfacing films in a manner that achieves substantially uniform conductivity across many surfacing films, resulting in conductivity discontinuities that may result in enhanced likelihood of damage during LS or ESD and/or impaired EMI shielding. In particular, metallic screens are further subject to corrosion, thermal expansion mismatch with the matrix that leads to micro-cracking, and impaired bonding with the matrix, each of which may further diminish the LS/ESD/EMI protection afforded by the surfacing film.